Viburnum leaf beetle (2001)
Pyrrhalta viburni
The first record of the viburnum leaf beetle (VLB) in North America was from Ontario in 1947, although it may have arrived in the early 1900s on nursery stock from Europe. In the U.S., this beetle was found in New York State in 1996. The beetle rapidly spread to neighboring states of Maine, New Hampshire, Vermont, and some of Pennsylvania and Ohio. In 2001, it reached British Columbia. WSU Master Gardeners found the first Washington specimens in Whatcom County a few years later. In 2015, there were reports of gardeners removing viburnums killed by this beetle. With good scouting and timing of pesticides, or winter shearing to remove eggs deposited at the tips of branches, it should be possible to retain viburnums in our home landscapes.
Pest description and damage The eggs of the viburnum leaf beetle are tucked into depressions chewed in the stem at the tips of the new growth. Eggs are covered with a mix of chewed plant tissue and excrement, highlighting the scars on twigs. The tiny larvae are greenish-yellow, becoming more yellow with rows of tiny black “pimples” as they mature. The pupa is small and white with some light brown on the top of the head, brown mouthparts and black eyes. The adults are drab brown and quite small (5-6 mm), but numerous on leaves. Damage to leaves is extensive and varies from brown skeletonized areas, areas of small holes between veins chewed by the older larvae, to oblong holes chewed by the adult. Trees and shrubs can be entirely defoliated. The greatest impact will be for nurseries, growers, arboreta collections, and mature landscapes.
Biology and life cycle VLB overwinters as eggs in the stems of the current season’s growth. In spring, they hatch and begin to scrape tissue from the underside of the leaf surface between the veins. As they grow, they are able to chew through the smaller veins leaving a network of holes. The mature larvae chew larger holes. Mature larvae crawl down the stem to pupate in the soil. The adults emerge and chew oblong holes in the remaining leaf tissue, leaving only the midrib and secondary veins. From egg to adult may take only two months. Adults mate and the females can lay up to 500 eggs. Adults continue feeding until the first hard frost.
Pest monitoring Look for, and remove, the distinctive egg scars on the most current stems during the winter when stems are bare. When infestations are heavy, it may take shearing to remove the branch tips. As the weather warms, check leaves near the tips of twigs for the first small larvae on the undersides of leaves. Next, small holes will appear in leaves. If you wait too long, the leaves will become ratty.
Management – cultural and physical
The VLB feeds on many Viburnum species: most susceptible include Viburnum dentatum (arrowood viburnums), V. opulus (European cranberry bush), V. opulus var. americana (American cranberry bush), V. rainesquianum (Rafinesque viburnum) and V. sargentii (Sargent viburnum). However, there are a number of viburnum cultivars that are resistant to VLB. These include resistant species such as V. plicatum var. tomentosum (doublefile viburnum), V. carlesii (Koreanspice viburnum), V. burkwoodii (Burkwood viburnum), V. x juddii (Judd viburnum), V. x rhytidiophylloides (lantanaphyllum viburnum) and V. rhytidiophylum (leatherleaf viburnum). For a complete list of resistant, moderately resistant, moderately susceptible to very susceptible viburnums see http://www.hort.cornell.edu/vlb/suscept.html.
Since larvae crawl down the stem (rather than drop) to pupate, a sticky barrier or up-facing v-shape tape collar around the trunk may provide some control. Hand-remove the larvae and adults in the morning hours before they warm up and become more active. Striking bushes with a stick over a paper box lid may dislodge beetles and larvae more quickly than handpicking. Populations may be reduced in warm winters when the eggs do not get their required prolonged chilling period.
Management – biological
While natural enemies specific to VLB are unknown in the U.S., generalist predators such as some birds, lady beetle adults and larvae, lacewing larvae and spined soldier bugs nymphs are known feed on larvae. The lady beetle adults and spined soldier bug adults also eat the adult viburnum leaf beetles. Ground beetles and insectivores also could feed on pupae in the soil and litter.
Management—chemical control
See “Leaf feeding beetles” in Table 2:
Chemical Control of Landscape Pests
For further information:
Department of Horticulture Cornell University. 2018. Managing Viburnum Leaf Beetles. http://www.hort.cornell.edu/vlb/manage.html
Murray, T., E. LaGasa, C. Looney, N. Aflitto. 2016. Pest Watch: Viburnum Leaf Beetle. WSU Extension Publication FS202E. https://pubs.extension.wsu.edu/pest-watch-viburnum-leaf-beetle-home-gard...